1. Field of the Invention
Natural gas has appeal as a vehicle fuel because of its economic and environmental advantages over gasoline. Emissions of reactive hydrocarbons, carbon monoxide, and particulate matter are very low from natural gas fueled engines, but catalysts are still necessary to meet emissions standards. This invention relates generally to methods for treating the exhaust of natural gas fueled vehicles in order to reduce the emission of methane and carbon monoxide and reduce nitrogen oxides in the exhaust gas. In particular, this invention relates to a two-part catalyst support system for palladium catalysts used in such methods.
2. Description of the Related Art
Catalysts for methane oxidation are known. C. S. Weaver discusses such catalysts in SAE paper #892133, "Natural Gas Vehicles--A Review of the State of the Art." S. H. Oh et al. in J. Catal., 132, 287 (1991) tested cerium-containing noble metal catalysts for methane oxidation. R. F. Hicks et al. tested Al.sub.2 O.sub.3 -supported and ZrO.sub.2 -supported noble metal catalysts for methane oxidation in J. Catal., 122, 280 (1990) and Pd/Al.sub.2 O.sub.3 catalysts in J. Catal., 122, 295 (1990). A method for reducing methane exhaust emissions using a platinum or platinum/palladium catalyst was taught in U.S. Pat. No. 5,131,224.